I. Field of the Invention
This invention relates generally to an integrated composite perovskite oxide heterostructure material design and a combinational film growth fabrication method, thereof, to enable property enhanced, miniaturized, highly tunable analog devices and/or charge mediated voltage controlled magnetic devices for RF/microwave communications, RADAR, and electronic warfare applications.
II. Description of Related Art
Radio Frequency (RF) and microwave (MW) frequency components are of immense importance for defense (and commercial) communications, RADAR, and electronic warfare systems applications. Thin film perovskite oxide high dielectric permittivity non-linear materials have attracted considerable attention for their potential application in voltage controlled tunable RF/MW devices, including, phase shifters, filters, delay lines, resonators, impedance matching networks, voltage controlled oscillators etc. because the (relative) dielectric constant (∈r) of the material can be adjusted with the application of an external electric field. Thin film barium strontium titanate (Ba1-xSrxTiO3 or BST) is considered to be one of the leading candidate materials to enable these device components. Barium strontium titanate (Ba1-xSrxTiO3 or BST) has strong non-linear response to an applied electric field, both in the paraelectric (PE) and ferroelectric (FE) states. However, in the FE state, there are losses associated with polarization switching. Therefore, for practical tunable device applications BST must be utilized in its PE state. Although the losses are lowered in the PE state, unfortunately so is the dielectric constant and its associated tunability. The current generation of tunable devices is based on uniform composition paraelectric BST films; hence there is a need to improve the material performance such that a high dielectric permittivity is attained without compromising other required performance metrics. At the same time there is a need to enhance dielectric response to create novel multiferroic hetero structure devices.
Maximizing the dielectric response (dielectric permittivity) of BST is critical for achieving high tunability, realizing RF/MW device miniaturization and for creating novel multiferroic heterostructures. Tunability promotes performance agility and high tunability (enhanced dielectric permittivity) is a desirable critical performance metric for tunable devices. The importance of miniaturization cannot be overemphasized for tunable RF/MW device technologies, especially for hand-held communications applications, and/or On-The-Move (OTM)/mobile communications systems both of which demand improved functionality with decreased size and weight. Additionally, to comply with future RF/MW systems architectures, which place stringent requirements on reduced size, weight, and power in concert with reduced component size and numbers the realization of miniaturized high performance voltage controlled BST-based devices is paramount. Maximizing the dielectric response/permittivity, of BST is critical for realizing RF/MW device miniaturization. For tunable BST-based RF/MW devices (e.g. resonators etc.) the size of the device at any particular resonant frequency depends on the inverse square root of the dielectric constant of the BST film, thus the larger the dielectric constant the smaller the device/component. Therefore, BST films with the highest possible dielectric permittivity would be optimal to ensure device miniaturization as well as enhanced tunability.
In addition to achieving high tunability and device miniaturization, it is also critical to innovate new and novel integrated multiferroic devices such as voltage controlled magnetic devices which would replace the current tunable and/or non-reciprocal devices used in present day RF/MW communications and RADAR systems. Commercial off-the-shelf (COTS) non-reciprocal components cost hundreds of dollars each and permanent magnets which are required to bias the magnetic domains of the magnetic medium make these MW components large and bulky. Voltage control of magnetism (converse magnetoelectric (ME) effect) is a critical technology to enable a new class of small size light-weight, low power integrated voltage tunable RF/microwave magnetic devices. Density functional theory (DFT) calculations has recently predicted a linear magnetoelectric (ME) effect, i.e., charge mediated voltage control of magnetism (VCM), for novel heterostructures composed of ferroelectric (FE)/ferromagnetic (FM) films, and that this effect is amplified by more than two-orders of magnitude if the dielectric/FE component (such as BST) were to possess a relatively large dielectric permittivity. A high dielectric permittivity perovskite oxide material, such as BST, would serve to facilitate more charge for a given applied field, hence enhance the magnetic response significantly to enable charge mediated VCM. Therefore, considering charge mediated VCM device applications, the BST thin film component must possess a high permittivity to enhance the ME effect.
However, for the above mentioned device scenarios, in addition to elevating the dielectric constant/permittivity of a film such as BST to achieve component miniaturization, high tunability and/or charge mediated voltage controlled magnetic devices; the BST based devices must still meet other crucial performance criteria. In specific, the enhancement of the dielectric permittivity must be accomplished while maintaining low loss, e.g., tan δ≦0.03 (low dielectric loss minimizes undesirable signal attenuation); low leakage current densities, e.g., 10−8-10−6 A/cm2 (high leakage currents limit device reliability severely restricting long term material/device operation and enhances power/battery draw); high break down field strength to ensure extended device operating reliability, and a smooth defect free surface morphology to promote reliable thin film integration and/or mitigate detrimental electrical device shorts. In addition the material process science fabrication methods must be complementary metal oxide semiconductor (CMOS) compatible to insure affordable IC integration, manufacturability and device/system affordability. In other words, for a thin film BST-based voltage controlled device to be useful and employed in engineering applications it must be commercially viable. To be commercially relevant thin film processes technologies must enjoy the economy of scale and affordability associated with the semiconductor industry. Thus the film fabrication process must possess the semiconductor industry-standard attributes to include, but not limited to: low cost large diameter substrate type (e.g., Si, GaAs/III-V, sapphire etc. with wafer diameters ≧4 inches); large area, cost and time effective film growth (sputtering, e-beam, CVD, dip/spin coating etc.); low to moderate process temperatures to facilitate materials integration and the required lithographic, etching, and passivation process steps (Tprocess≦800° C.); and compatibility with foundry cluster tools (e.g., wafer size and temperature requirements, utilize standard growth tools).
Various methods for enhancing the permittivity of FE thin films are proposed. As one of them, an extremely high dielectric constant, ∈r=7000 at 10 GHz, has been achieved for SrTiO3 films grown by reactive molecular beam epitaxy (MBE) on DyScO3 substrates. The enhanced permittivity is due to the fact that the single crystal DyScO3 substrate promotes the growth of SrTiO3 films under uniform biaxial tensile strain which in-turn causes the FE polarization to lie in the plane of the film, thereby elevating the films permittivity. Although this is a successful approach to achieve a high permittivity, the expensive MBE growth technique combined with the expensive designer substrate (DyScO3) which is size and availability limited, makes this approach non-industry standard (i.e., not integratable/compatible with semiconductor technology) and cost prohibitive. Other concepts to achieve high permittivity BST films involve attaining an engineered structural distortion within the BST film. An in-plane tetragonally distorted film (c<a) has been achieved by varying the Ar:O2 ratios for pulsed laser deposited (PLD) Ba0.5Sr0.5TiO3 (BST50/50) films on (001) MgO substrates. The high permittivity (∈r=1200) for in-plane distorted films, results from the fact that the in-plane ionic polarization is enhanced to form, and develop, with an in-plane applied electric field. Unfortunately the fact that PLD is a non-standard and small area film deposition technique and that MgO is a small area expensive substrate suggests this approach to be very useful for only laboratory scale experiments but not viable from the affordability and manufacturability/foundry points of view. In addition utilization of an in plane electric field is only possible for device structures which are co-planar device designs (i.e., no bottom electrode) and is not viable for the metal-insulator-metal (MIM) configuration which is required for the charge mediated voltage control of magnetic devices and most tunable varactor device designs. Moderately high permittivities (∈r=526) have also been achieved via sputter deposited BST 50/50 films on “surface modified” MgO substrates. The “surface modification” of the MgO substrates involves a pre-anneal process step at 1145° C. for 5 hours prior to the BST growth. Although effective in elevating the BST permittivity, the results are achieved for films grown on non-industry standard, expensive, small area MgO substrates, and this approach introduces added complexity via a high temperature extended temporal duration substrate annealing step, hence this technique is not industry standard and/or cost effective. Other practices to enhance the permittivity (∈r≧300) of BST films involves employing thin (˜30 nm) physical vapor deposited (PVD) BST/self-buffer layer between the BST active thin film and the support substrate (usually Pt—Si) whereby the buffer layer is grown at a higher temperature (˜700° C.) versus that of the BST over-layer film (700° C.). For this situation/scenario the films' dielectric loss and leakage current characteristic are degraded and the films' surface morphology possessed pin-hole defects and extreme roughness (Rrms>10 nm). The high leakage current characteristics ultimately degrade the device long-term reliability and the rough defect laden surfaces would inhibit the integration of the BST with other films (i.e., metal electrodes and/or overlying FM film etc.).
Thus from the prior art summarized herein the achievement of thin film BST which possess an enhanced dielectric permittivity in concert with the other required performance and manufacture criteria is not easily achieved. Therefore there is a continuing need to develop an integrated material design and/or film growth fabrication methodologies which enables the simultaneous enhancement of dielectric permittivity in concert with low loss, low leakage current densities, high break down field strength, and a smooth defect free surface morphology. In addition the material process science methods must be complementary metal oxide semiconductor (CMOS) compatible to insure affordable IC integration, manufacturability and device/system affordability. Such an invention would enable a new class of tunable device components to realize the next generation of RF/MW communications, RADAR, and electronic warfare systems.